1. Field of the Invention
This invention relates to combatting certain yellows diseases in plants.
2. Description of the Prior Art
The "yellows" are a well known group of plant diseases occurring in temperate and tropical regions. They are found in over 300 genera of plants including monocotyledons and dicotyledons, trees and herbaceous plants. Yellows diseases have caused serious economic losses in long-term crops, for example peach, cherry, pear, citrus fruits and coconuts. Lethal yellowing of palms has wiped out the Jamaican coconut industry and has devastated plantations in Haiti, Cuba, Ghana, Nigeria and Florida, U.S.A. The disease aster yellow is particularly destructive and has caused major losses of crops in the U.S.A., including lettuce, carrots, celery, potatoes and flax. In continental Europe the apple proliferation disease is causing considerable problems.
Most of the yellows diseases are associated with Mycoplasma-like organisms (MLO). The term "Mycoplasma-like organism" or MLO, wherever occurring in this specification, is used in a broad sense to cover yellows disease-associated organisms of the family Mycoplasmataceae or Spiroplasmataceae. The term "Mycoplasma" is used in a narrower sense to refer to the genus of the same name with the family Mycoplasmataceae. The following chart indicates classification:
______________________________________ CLASS: MOLLICUTES ______________________________________ Order: Mycoplasmatales. Family I: Mycoplasmataceae Genus I: Mycoplasma Genus II: Ureaplasma Family II: Acholeplasmataceae Genus: Acholeplasma Family III: Spiroplasmataceae Genus: Spiroplasma Species: Spiroplasma citri Corn stunt spiroplasma and many others not yet fully characterised ______________________________________
MLO are very small organisms, of diameter typically from 300 nm to 1 micron, and are the smallest known cells capable of independent multiplication. Instead of having a rigid cell wall like bacteria they have a triple-layered plasma membrane typically 7-11 nm thick. MLO occur in the phloem of plants but not in other parts of the plant tissue. While mycoplasmas are of variable shape, spiroplasmas adopt a helical shape when constrained to do so, e.g. by osmotic pressure effects when they are contained in plant phloem.
Until the late 1960s it was thought that yellows diseases were caused by viruses. In the 1970s many of them were shown to be associated with MLO. Attempts have been made to treat the disease by use of chemicals, for example tetracycline. In a paper given by Markham to the British Crop Protection Conference in 1977, a review was made of possible future trends in combatting yellows diseases. It was suggested that chemical control and the breeding of resistant MLO were the most promising approaches.